Hydrographic Survey Standards in Nigeria PDF

HYDROGRAPHIC SURVEY STANDARDS IN NIGERIA CO UNCIL O R...

HYDROGRAPHIC SURVEY STANDARDS IN NIGERIA CO UNCIL O R FN SURVEYO IGERIA YA ND FAITH PEACE AND PROGRE UNIT SS SURVEYORS COUNCIL OF NIGERIA (SURCON) PRINTED BY 08060012939  HYDROGRAPHIC SURVEY STANDARDS IN NIGERIA CO UNCIL O R FN SURVEYO IGERIA YA ND FAITH PEACE AND PROGRE UNIT SS SURVEYORS COUNCIL OF NIGERIA (SURCON) 49 48  FORWARD The need to tap the natural resource of the solid surface below water bodies has given tremendous impetus to the study and application of hydrography. Thus the accurate knowledge of the characteristics of the water body, the hard surface below the water body and the coastlines are essential to the use and exploitation of the resources of the water bodies. Hydrographic Surveying is a highly skilled profession where the expertises of both local and expatriate companies are applied. To harmonize the procedures and practices of the organisations engaged in hydrographic surveying in Nigeria, the Surveyors Council of Nigeria has put together the specifications that will guide the hydrographic operation. This is done by the Council to fulfill its statutory responsibility of regulating and controlling all aspects of surveying practice in Nigeria. The specifications are drawn from the inputs and experiences of some organisations already operating in Nigeria and the required international standards. Furthermore the guidelines were subjected to a stakeholders workshop where more rewarding inputs were accessed. The specifications and guidelines have been duly approved by the Surveyors Council of Nigeria (SURCON). It is therefore mandatory that all surveyors lawfully qualified to carry out hydrographic surveying in Nigerian waters must adhere to the procedures and methods stipulated in the booklet. Surv. (Prof) R.N. Asoegwu fnis President SURCON 47 i  PREFACE On behalf of the president and members of the Surveyors Council of Nigeria (SURCON). I present this specification for Hydrographic Surveys to the public especially the major stakeholders. And at the same time I thank all who helped in the production of this document. The production of this specification went through an expert review process, a stakeholders workshop before the final approval by the Surveyors Council of Nigeria (SURCON). However we are still open to comments and reaction that will help us in the future review of the Specifications. This Specification for Hydrographic Surveys will go a long way in charting a (sea) route of standards for the practice of hydrographic surveys in Nigeria. This is in keeping with the mandate of SURCON as encapsulated in the Enabling Act, Cap 425 of the Laws of the Federation of Nigeria with particular reference to section 4, subsection (d). I therefore urge all to go beyond the possession of this document Specification for Hydrographic surveys and apply all the specifications in their practice of hydrographic surveys in Nigeria. Thankyou. Surv. C. I. Nwabichie, fnis, J.P Chairman Survey Laws & Regulations Committee. ii 46  TABLE OF CONTENTS Forward.............................................................................................................i Preface...............................................................................................ii 1.0 Introduction.............................................................................1 2.0 Technical Specifications.......................................................2 2.1 Classification of Surveys................................................:....2 2.2 Positioning Systems..............................................................5 2.2.1 System Positioning...............................................................5 2.2.2 Calibration............................................................................6 2.3 Processing of Position Data..................................................6 2.3.1 Survey Computer..................................................................6 2.3.1.1 Data Acquisition...................................................................7 2.3.1.2 Data Processing....................................................................7 2.4 Horizontal Controls..............................................................7 2.4.1 Datum and Ellipsoid.............................................................7 2.4.2 Primary shore Stations..........................................................7 2.4.3 Hydrographic Signals...........................................................8 2.4.4 Position Fixes and Floating Aids..........................................8 2.4.5 Aids to Navigation................................................................8 2.4.6 Offshore Installations Dangerous to Navigation..................8 2.4.7 Positioning of Soundings......................................................8 2.5 Vertical Controls...................................................................9 2.5.1 Sounding Datum and Chart Datum......................................9 2.5.2 ` Tidal Observation.................................................................9 2.6 Data Acquisition.................................................................10 2.6.1 Sounding.............................................................................10 2.6.1.1 Requirement........................................................................10 2.6.1.2 Characteristics....................................................................10 2.6.1.3 Calibrations.........................................................................10 2.6.1.4 Depth Measurement............................................................10 2.6.1.5 Accuracy...............................................................................II 2.6.1.6 Interval between Sounding lines...........................................II 2.6.1.7 Interval between Plotted Soundings...................................12 2.6.1.8 Spacing of Positions Fixes..................................................12 2.6.1.9 Recommended Tracks.........................................................12 2.7 Sonar Sweeping..................................................................12 2.7.1 Sonar Records.....................................................................12 2.8 BottomlWater Sampling.....................................................13 2.8.1 Magnetometer Search.........................................................13 2.9 Rendering of Data...............................................................13 2.10 Draughting..........................................................................13 2.11 Chart Drawing....................................................................14 3.0 Dredging.............................................................................14 3.1 Hydrographic Survey Specification for Dredging..............14 4.0 Surveys for Measurement Purposes...................................16 5.0 Computation of Quantities..................................................16 6.0 Data Attribution..................................................................17 6.1 General................................................................................17 45 iii 6.2 Point Data Attribution.....................................................................................17 7.0 Search.............................................................................................................17 7.1 Introduction....................................................................................................1 7 7.2 Extent o fAre a to be Searched.......................................................................17 7.3............. Conducting the Search.................................................................................... 1 X 7.4 Presentation of Search Results...................................................................1 X 8.0 Guidelines for Quality Control.....................................................:..........1 X 8.1 Introduction................................................................................................1 X 8.2 Positioning............................................................................,...................1 X 8.3 Depths..........................................................................................................IX 8.4 Sounding Density.........................................................................................18 8.4.1 Single beam Echo Sounders.......................................................................1 8 8.4.2 Sound Scan Sonar.........................................................................................I 8 8.4.3 Multibeam EchoSounder.............................................................................19 8.4.4 Sweep Systems............................................................................................19 8.4.5 Air borne Laser............................................................................................19 8.4.6 Geostatics.. ;................................................................................................19 8.5 Error Source and Budget..............................................................................20 9.0 Presentation of Data.....................................................................................20 PART 2.1 POSITION..................................................................................................22 2.A.1 General....................................................................,....................................22 2.A.2 Type Category..............................................................................................22 2.A.3 Accuracy Category.......................................................................................22 2.A.4 Notes on Positions.....................................,.........................................,......23 2.A.5 Example.......................................................................................................23 PART 2.B SOUNDINGS..............................................................................................23 2.B.1 General.........................................................................................................23 2.B.2 Type Category........................................'"...................................................23 2.B.3 Accuracy Category.......................................................................................24 2.B.4 Example.......................................................................................................24 PART 2.C......... FIDELITY WITH WHICH SCALED SOUNDINGS.........................REPRODUCE SEABED...............................................................................24 2.C.l.....................General...........................................................................................................24 2.C.2....................Type Category.....,.........................................................................................24 2.C.3....................Accuracy Category.........................................................................................24 2.C.4.....................Example.........................................................................................................25 PART2.D.............DATAPROCESSING.................,..................................................................25 2.D.1....................General...........................................................................................................25 2.D.2....................Type Category................................................................................................25 2.D.3....................Accuracy Category.........................................................................................25 2.D.4....................Example.........................................................................................................25 Collective Example......................................................................................................................26 Definitions...............................:.............................................................................................26-38 References....................................................................................................................................39 iv 44  LIST OF FIGURES Figure 1: (a)TheCoasterzone...........................................................................................................40....................(b) The Surf or breaker zone..............................................................................................40 Figure 2: Seasonal Cycle of a beach caused by differing wave Conditions....................................41 Figure 3 : (a) Dissipative mild sloping beach with spilling breakers................................................41...................(b) Intermediate beach with plunging and Secondary breakers.........................................41....................(c) Steep sloping reflective beaches..................................................................................41 Figure 5: Features associated with accreting coasts..........................................................................42 Figure 6: Longshore drift of sand on the beach face and by a long shore current...................within the zone surf............................................................................................................43 Figure 7: The elements of a rip Current embayment.........................................................................43 Figure 8: Schematic of the principal Component involved in the development...................of a littoral cell sediment budget.......................................................................................44 Figure 9: Sediment deposits usually become thinner away from the source area, and sediment...................grains become finer and more rounded...............................................................................44 Figure 10: Typesofsurfacewaves.........................................................................................................45 Figure 11: Type of tides.......................................................................................................................46 Figure 12: A vertical profile.................................................................................................................47 Figure 13: Ware refraction on an irregular coast................................................................................47 Figure 14: The velocity profile for a steady current flow over a bed..................................................48 Figure 15: For cadosarea Datum..........................................................................................................49 LIST OF TABLES Table 1 :......Summary of minimum standards for hydrographic surveys................................................4 Table 2:.......Summary of Minimum Standard for Positioning of Navigational Aids and.....................Important features...............................................................................................................9 Table 3:.......Bathymetric Model Accuracy...........................................................................................19 Table 2.B.3 Accuracy Category 43 v  HYDROGRAPHIC SURVEY STANDARDS IN NIGERIA SPECIFICATIONS FOR HYDROPGRAPHIC SURVEYS IN NIGERIA 1.0 INTRODUCTION Hydrographic surveying is undergoing fundamental changes in measurement technology. Multibeam acoustic and airborne laser systems now provide almost total seafloor coverage and measurement as compared to the earlier sampling by bathymetric profiles. The capability to position the data precisely in the horizontal plane has increased enormously by the availability or satellite positioning systems, particularly when augmented by differential techniques. This advance in technology has been particularly significant with navigators now able to position themselves with greater accuracy than that of the data on which charts are based. It should be noted, however, that the accuracy and completeness of a hydrographic survey can never reach that of land mappmg. The increased use of satellite positioning.ems by the mariner, combined with the cost effectiveness and improved accuracy provided by these systems (over more traditional terrestrial- based precise navigation systems), have encouraged hydrographics agencies to utilise systems that afford positioning accuracy equal to or better than those enjoyed by the mariner for all future surveys conducted in Special Order and Order 1 (see table 1). The required positioning accuracies are to a large extent based on the practical limitations of draughtsmanship at a given scale. Automated data management allow~ata to be presented at any scale. Therefore the accuracy requirements for positions must be a function of the errors contributed by positioning and sounding systems and the likely use of the data. The scope of the Hydrographic Surveying practice referred to in this specification extends from port surveying including inland waterways to National charting and oceanographic data processes. In limited cases, it may includes water-training process using dams and irrigations works. The National charting and surveying includes collection and processing of oceanographic and hydrographic data with the primary objective of ensuring the safety of navigation. The objective and scope of the port and Inland Waterways surveys are to provide variety of survey services implicit in every stage of port conservancy, port development as well as improving agricultural/Mineral processes and distribution. In addition, hydrographic surveys are needed in water resources management, dam construction and monitoring, irrigation, industrial fishing, recreational activities, hydroelectricity, navigation, flood and lake management, and marine pollution. Other areas where hydrographic surveys are needed are oil exploration and exploitation activities, and oil spill management. The scope and precision of the survey measurements and the variety of options to be considered in arriving at economic specification are related to current knowledge and improvement on technology. The planning for each hydrographic surveying and the preparation of appropriate specifications is a unique task and it is not possible to prepare a treatise on accuracy standards for hydrographic 1 42  Surveys which would be applicable for any area to be surveyed. The density of sounding and the precision of measurements depend on several factors; the depth of water, the composition and configuration of the bottom and the type of equipment in use. All need to be considered. The state of the art of the depth measurement equipment has been evaluated as follows: a) Single beam echo sounders have reached a sub-decimetre accuracy in shallow water. The market offers a variety of equipment with different freqv-ncies, pulse rates etc. and it is possible to satisfy most users' and, in particular, the hydrographers' needs. b) Side scan sonar equipment technology has also reached a high level of bottom obstacle detection and definition. Although, at present, its use is limited by the low speed (5-6 knots maximum) at which it can be operated, it is widely employed for harbour and navigable channels surveys to ensure obstacle detection between the measured survey lines. Many hydrographic agencies consider its use compulsory in such areas, often prescribing overlaps of 1 00% or more. c) Multibeam echosounder technology is developing rapidly and offers great potential for accurate and total seafloor search if used with proper procedures and provided that the resolution of the system is adequate for proper detection of navigational hazards. d) Airborne laser sounding is a new technology which can offer substantial productivity gains for surveys in shallow, clear water. Airborne laser systems are capable of measuring depths to 50m or more. It is likely that many hydrographic surveys will continue to be conducted with single beam echo sounders which only sample discrete profiles of the seafloor, with the 100% bottom search techniques outlined above possibly only employed in critical areas. This assumption led to the decision to retain the concept ofline spacing even though it is no longer directly related to survey scale. Equipment and procedures used to achieve the standards laid down in this paper are left to the discretion of the agency responsibly for the survey quality. The optimum results are achieved when the appropriate procedures and equipment are used in conjunction with the expertise and training of the hydrographic surveyor. The importance of professional judgement cannot be overemphasized. The Hydrographic Surveying Specifications outlined below are developed essentially to assist in general terms. 2.0 TECHNICAL SPECIFICATIONS 2.1 Classification of Surveys To accommodate in a systematic manner different accuracy requirements for area to be surveyed, four orders of survey are defined. These are described below and in Tables land 2 which summarize the overall requirements and are in fact the essence of the complete standard. Special Order Special Order hydrographic surveys approach engineering standards and their use is intended to be restricted to specific critical areas with minimum underkeel clearance and where bottom characteristics are potentially hazardous to vessels. These area have to be explicitly designated 41 2 by the agency responsible for survey quality. Examples are harbours, berthing areas, and associated critical channels. All error sources must be minimized. Special Order requires the use of closely spaced lines in conjunction with side scan sonar, multi-transducer arrays or high resolution multi beam echosounders to obtain 100% bottom search. It must be ensured that cubic features greater than 1 m can be discerned by the sounding equipment. The use of side scan sonar in conjunction with a multibeam echosounder may be necessary in areas where thin and dangerous obstacles may be encountered. Order 1 Order 1 hydrographic surveys are intended for harbours, harbour approach channels, recommended tracks, inland navigation channels, and coastal area of high commercial traffic density where underkeel clearance is less critical and the geophysical properties of the seafloor are less hazardous to vessels (e.g. soft silt or sand bottom). Order 1 surveys should be limited to areas with less than 100m water depth. Although the requirement for saeafloor search is less stringent than for Special Order, full bottom search is required in selected areas where the bottom characteristics and the risk of obstrucrtions are potentially harzardous to vessels. For these areas searched, it must be ensured that cubic features greater than 2m up to 40m water depth or greater than 10% of the depth in areas deeper than 40m can be discerned by the sounding equipment. Order2 Order 2 hydrographic surveys are intended for areas with depths less than 200m not covered by Special Order and Order 1 and where a general description of the bathymetry is sufficient to ensure there are no obstructions on the seafloor that will endanger the type of vessel expected to transit or work the area. It is the criteria for a variety of maritaime uses for which higher order hydrographic surveys cannot be justified. Full bottom search may be required in selected areas where the bottom characteristics and the risk of obstructions may be potentially hazardous to vessels. Order3 Order 3 hydrographic surveys are intended for all areas not covered by Special Order, and Orders 1 and 2 in water depths in excess of 200m. Notes: - For Special Order and Order 1 surveys the agency responsible for the survey quality may define a depth limit beyond which a detailed investigation of the seafloor is not required for safety of navigation purposes. - Side scan sonar should not be used for depth determination but to define areas requiring more detailed and accurate investigation. 3 40 REFERENCES Beatley, Timothy, David J. Brower and Anna K. Schwab. An Introduction to Coastal Zone Table 1: Summary of Minimum Standards for Hydrographer Surveys Management. Island Press, 1994. Brown, Joan, Angela Colling, Dave Park, John Phillips, Dave Rothery and John Wright. Wave,s ORDER SPECIAL 1 2 3 Tides and Shallow-water Processes. Pergamon, 1994. Delft Institute of Hydraulic Engineering. Glossary of Coastal Engineering Terms. Found at Examples of Herbours, Harbours, harbour Areas not Offshore areas htttp://www.ihe.nl/he/topics/glossary.htm Typical A rea berthing approach L;.ennels, described in not described in Hardisty, J. Beaches: Form and Process. Unwin Hyman Ltd., 1990. areas, and Recommended Special Special Order, associated tracks and some Order and and Orders 1 International Hydrographic Organisation. IHO Standards for Hydrographic surveys international critical Coastal areas with Order 1, or and 2 Hydrographic Bureau, 1998 channels depths up to 100m areas up to King, Cuchaine A.M Beaches and Coasts. EdwardArnold, 1961. with 200m water Komar, Paul D. Beach Processes and Sedimentation. Prentice-Hall, Inc., 1996... rrummum depth National Geodetice Survey. Geodetic Glossary. U.S. Department of Commerce, 1986. underkeel Pickard, George L. And William J. Emery. Descriptive Physical Occeanography. Pergamon Press, clearances 1990. Horizontal Accuracy 2m 5m + 5% of depth 20m+ 5% 150m+5%of Pipkin, Bernard W., Donn S.Gorsline, Richard E. Casey and Douglas E. Hammond. Laboratory (95% Confidence of depth depth Exercises in Oceanography. W.H. Freeman and Company, 1977. Level) Plummer, Charlse C. And David McGeary. Physical Geology. Wm. C. Brown Publishers, 1985. Depth Accuracy a=0.25 m a = 0.5 m a = 1.0 m Same as Oder 2 Shepard, Francis P. Submarine Geology. Harper & Row, 1963. for Reduced b = 0.007 b=0.013 b = 0.023 U.S. Army Corps of Engineers. Coastal Geology. EM 1110-2-1810,31 January 1995. Depths (95% U.S. Army Corps of Engineers. Miscellaneous Paper No. 2-72. April 1972. Confidence Level) (I) U. S. Army Corps of Engineers. Miscellaneous Paper No.2 -7 4. March 1974 U.S. Fish and Wildlife Service. Biological Impact of Minor Shoreline Structure on the Coastal 100% Bottom Compulsory Required in Maybe Not applicable Environment: State of the Art Review. Volume 1, 1980. Search (2) selected areas (2) required in selected Voigt, B. Glossary of Coastal Terminology. Washington State Department of Ecology, Coastal areas Monitoring & Analysis Program, Publication No. 98-105, 1998. -. Whitten, D.G.A. The Penguin Dictionary of Geology. Penguin Books, 1979. System Detection Cubic Cubic features> Same as Not applicab~e Wiegel, Robert L. Waves, Tides, Currents and Beaches: Glossary of Terms and List Standard Capability features> I 2m Order 1 Symbols. Council on Waves Research, July 1953. m in depths up Wright, L. Donelson. Morphodynamics of Inner Continental Shelves. CRC Press, Inc., 1995. to 40m; Www.cse.noaa.gov/themes/nsdi/nsdi.pdf 10% of depth beyond 40 m (3) Maximum Line Not applica- 3 x average depth 3-4 x average 4 x average depth Spacing (4) ble, as 100% Or 25m, whichever depth or search is greater 200m, compulsory whichever is greater 39 4  To calculate the error limits for depth accuracy the corresponding values of a and b listed in Wave length: The distance, in meters, between equivalent points (CREST or TROUGHS) on waves. Table I have to be introduced into the formula ±v?[ a) Capabilities faulting, landsliding, or volcanic activity); also called seismic sea wave. Commonly misnamed The Computer shall be interfaced for simultaneous acquisition in real time of all TIDAL WAVE. See Figure 10 _ data from the positioning system (s) and survey sensors. Water depth: Distance between the tine seabed and the still water level. Accept manual entry of all the data as may be necessary. Water level: The ELEVATION of a particular point or small patch on the surface of a body of water above a specific poi(it or surface, averaged over a period of time sufficiently long to remove b) Cycle Time the effects of short period disturbances. This is the total processing time for data acquisition, computation.display, logging, Wa.te line: (1) The jurcture of land and sea. This line migrates, chan~in~ w~th the tide or other printing and plotting (if required) of the survey data. This shall not exceed 5 vanation of the water leve I. Where waves are present on the BEACH, this hne IS also known as the seconds. limit of BACKRUSF 1. (2) The common boundary between the water surface and any immersed C) Offsets structure. Computed co-ordinaties shall refer to the positioning system antennae or to offset Water mark: A line or mark left on the shores of a body of' water by the water as an indication ofthe position (with full diagrams). The computer shall allow for the following: water's former ELEVATION. i) One primary offset; Water, navigable Mfi: the waters which are or can be used as water highways for commerce. ii) three secondary offsets; Wave: (I) An oscillatory movement in a body of water manifested by an alternative rise and fall of iii) Antennae heights and offsets in the event of one or more secondary positioning the surface. (2,) A disturbance of the surface of a liquid body, as the ocean, in the form of a ridge, systems being used. swell or hurnp. (3) The term wave by itself usually refers to the term SURFACE GRAVITY WAVE (PROGRESSIVE). 2.3.1.2 Data Processing Wave crest; (I) The highest part of the wave. (2) That part of the wave above still water level. a) Capabilities Wave direction: The direction from which the waves are coming. For each positioning systems in use, the computer shall be capable of but not Wave height: The vertical distance between the crest (the high point of a wave) and the through limited to the following: 'Ie low po int). 37 6  i) Selection of primary and secondary position systems. damage by wave action. It retains earth against its shoreward face. (2) (SMP) A structure separating ii) Correction for zero/delay constant, (COTS) and system propagation velocity. land and water areas primarily to prevent EROSION and other damage by wave action. Generally more massive and capable of resisting greater wave forces than a BULKHEAD. iii) Computation of the co-odinates of the fix shall be based on natural co-ordinates system iflarge area of chart is involved i.e. Small scale charts and UTM plane co- Semidiurnal tide: Tides occurring twice daily. There are two high and two lows per tidal day. ordinates system for large scale charts. Shore: That strip of ground bordering any body of water which is alternatively exposed, or convened by tides and/or waves. A SHORE of unconsolidated material is usually called a BEACH. iv) Flag when the angle of cut between position lines does not meet Technical Specifications. Shoreline: (I) The intersection of a specified plane of water with the shore (2) (SMP) All of the water areas of the state, including reservoirs and their associated uplands, together with the lands xxii) Computations of the standard deviation offix shall be based on the residuals of the underlying them, except those areas excluded under RCW 90.58.030(2)( d). least square adjustment. Slack water (slack tide): The state of a tidal current when its velocity is near zero, especially the vi) The computation of the co-ordinates should be related to the designated primary moment when a reserving current changes its direction and its velocity is zero. The term is also system, if one or more secondary positions system( s) are in use. applied to the entire period of low velocity near the time of turning of the current when it is too i) Data Output weak to be of any practical importance in navigation. The relation of the time of slack water to the Data output shall be a chart conforming to standard specifications which shall tidal phases varies in different localities. In some places slack water may occur midway between include among others, the following: Projection Grid and Height, Atum, Ground high and LOW WATER. Controls, Scale, Contours and Spot Heights, Map Details, Feature Codes, Border, Depths, Navigational Details,Aids, Bouys Hazards, Wrecks in acceptable colours Sounding: A measured DEPTH of water. On hydrographic charts the sounding are adjusted to a and Authors and References, etc. specific plane of reverence (SOUNDING DATUM). Sounding Datum: Sounding Datum is the level to which soundings are reduced in course of 2.4 Horizontal Controls 2.4.1 Datum and Ellipsoid hydrographic survey and is therefore the datum used for the completed fair chart or final tracing. The geodetic datum to be used for any survey will be that already adopted for geodetic work in the Sounding line: A line, wire or cord used in SOUNDING. it is weighted at one end with a plummet. country. But in some cases, the local datum can be adopted if it is not possible to tie it to the national Spring tides: A tide that occurs at near the time of new or full moon, and which rises highest and datum. This will include the ellipsoid to be used. This information will normally be got from falls lowest from the MEAN SEA LEVEL (MSL) Federal Surveys or the Hydrographerofthe Navy. Storm surge: A rise or piling-up of water against shore, produced by strong winds blowing Datum without specific titles should be described in full in the Geodetic Data, i.e. ONSHORE. A storm surge is most severe when it occurs in conjunction vith a high tide. Geographical position of the origin; See Figure 10. Azimuth used to control the geodetic scheme; Separation of the Geoid and Ellipsoid at the point of origin; Surge: (1) Long-interval variations in velocity and pressure in fluid flow, no necessarily periodic, Deviation of the vertical at the point of origin perhaps even transient in nature. (2) The name applied to wave motion win a periodic, perhaps Ellipsoid used. even transient in nature. (2) The name applied to wave motion with a period in~rmediate between that of an ordinary wind wave and that of the tide. (3) Changes in water leTel as a result of Horizontal control shall be established by triangulation, traverse, trilateration or satellite observation or the combination of these techniques. Generally, the accuracy of the horizontal meteorological forcing (wind, high or low barometric pressure) causing a differtlce between the control shall be preferably 3rd order (1: I 0,000) or higher where the coverage is extensive. recorded water level and that predicted using harmonic analysis, may be positive or1egative. 2.4.2 Primary Shore Stations Swell: Waves that have traveled a long distance from their GENERATING AREA ,rrl have been The location of primary shore control stations and electronic positioning stations shall be not less sorted out by travel into LONG WAVES ofthe same approximate period. than 3rd order control when the geodetic net of height order used as the origin. Primary shore Tidal current: The alternating horizontal movement of water associated with the ris- and fall of the control points should be located by ground survey methods to a relative accuracy of I part in tide caused by ASTRONOMICAL TIDE-producing forces. ~ 100,000. When geodetic satellite positioning methods are used to establish such points, the error Tidal period: The interval of time between two consecutive like phase ofthetide or tidacurrent. Tidal Waters: Tidal waters are those bodies of water, which are affected by tide-producing ttrces. The tidal waters may be affected by diurnal forces or semi diurnal forces. Tidal Wave: (1) A wave, in the oceans and sea, produced by tides and tidal currents. (2) NOI, 7 36 Pile: A long substantial pole of wood, concrete or metal, driven into the earth or sea bed to serve as should not exceed 10 em at 95% confidence level. When the extent of the geodetic survey is in a support or protection. excess of 50km, second order control method will be used, and if the stations of an electronic Precision: A statistical measure of repeatability of a value, usually expressed as variance or positioning system are separated by distances in excess of200km, ties shall be made to basic first standard deviation of repeated measurements. order control whenever possible. Beacons may be built in addition to primary geodetic controls or Radar: An instrument for determining the distance and direction to an object by measuring the metal or plastic pipes of minimum diameter of 50mm inserted into a concrete mix of 1 :2:4. time needed for radio signals to travel from the instrument to the object and \back, and by rd Concrete portion ofthe beacon shall be 30 x 30 x 150cm, and 40cm above ground level, for 3 order measuring the angle through which the instrument's antenna has traveled.. as specified and located above the high water mark (HHWM) or in special circumstances a shorter Range of tide: The difference in height between consecutive high and low waters. The MEAN beacon and a long pipe may be employed (30 x 30 x 50 and 20cm above the ground level). Each RANGE is the difference between MEAN HIGH WATER and MEAN LOW WATER. The beacon must be witnessed by' at least 4 No. Pillars marked A, Band C and D measuring 15 x 15 x GREAT DIURNAL RANGE or DIUNAL RANGE is difference in height between MEAN 50cm established at about 100m apart id a direction away from line of erosion. The beacons so HIGHER HIGH WATER (MHHW) and MEAN LOWER LOW WATER (MLLW). Where the established must be accompanied with a proper station description and recovery report. type of tide is DIURNAL, the mean range is the same as the DIURNAL range. See Figure 11. Reference Station: A tide or current station for which tidal or tidal current constants have 2.4.3 Hydrographic Signals previously been determine and which is used as a standard for the comparison of simultaneous The error in location of hydrographic signal used for visual fixing, with relation to the primary observations at a second station; also a station for which independent daily predictions are given shore control should not exceed 1 mm at the scale of the survey. in the tide or current tables from which corresponding predictions are obtained for other station by means of differences or factors. 2.4.4 Position Fixes and FloatingAids Revetment: (1) A facing of stone, concrete, etc., To protect an EMBANKMENT, or shore The indicated repeatability of a fix (accuracy oflocation referred to shore control) in the operating structure, against EROSION by wave action or current. (2) A retaining wall. (3) (SMP) Facing of area, whether observed by visual or electronic methods, combined with the plotting error, shall at stone, concrete, etc., Built to protect a SCARP, EMBANKMENT or shore structure against the scale of the survey not exceed 1.5mm. erosion by waves of CURRENTS. Ocean Surveys for nautical charts (shoal searches, investigation of doubtful soundings, etc) Rip current: A strong surface current of short duration flowing seaward from the shore. It usually acceptable error when fixing a reference beacon by astronomic or electronic/satellite method of 1 appears as a visible band of agitated water and is the return movement of water piled up on the km Range (O.Sm). shore, by incoming waves and wind. Arip current consists ofthree parts: the FEEDER CURRENT flowing parallel to the shore inside the BREAKERS; the NECK, where the FEEDER 2.4.5 Aids to Navigation CURRENTS converge and flow through the beakers in a narrow band or rip; and the HEAD, a) Fixed aids to navigation shall be located within the same limits of accuracy as where the current widens and slackens outside the breaker line. See Figure 7. primary shore station stated in paragraph 2.4.2 above. Rotary current tidal: A tidal current that flows continually with the direction of flow changing through all points of the compass during the tidal period. Rotary currents are usually found b) Floating aids to navigation shall be located within the same limits of accuracy offshore where the direction of flow is not restricted by any barriers. The tendency for the rotation as position fixes stated in section 2.4.4 above. in direction has its origin in the deflecting force ofthe earths rotation and, unless modified by local conditions, the change is clockwise in the Northern Hemisphere and counterclockwise in the 2.4.6 Offshore Installations Dangerous to Navigation Southern Hemisphere. The velocity of the current usually varies throughout the tidal cycle. Location of offshore installation (e.g oil rigs) dangerous to navigation should meet the requirement Passing through two maxima in approximately opposite directions and the two minima with the for third order control. direction of the current at approximately ninety degrees from the direction at the time of maximum velocity. 2.4.7 Positioning of Soundings Quality assurance: All those planned and systematic actions necessary to provide adequate The position of soundings, dangers, and all other significant submerged features should be confidence that a product or a service will satisfy given requirements for quality. determined such that the horizontal accuracy is as specified in Table 1. The accuracy of the position of a sounding is the accuracy at the position of the sounding on the bottom located within a geodetic Quality control: All procedures which ensure that the product meets certain standards and reference frame. The exception to this are Order 2 and Order 3 surveys using single beam echo specifications (IHO S32ed. 1994, #4115) sounders where it is the accuracy of the position of the sounding system sensor. In such cases, the Sea defenses: Works to prevent or alleviate flooding by the sea. agency responsible for the survey quality should determine the accuracy of the positions of Sea level rise: The long-term tread in MEAN SEAL LEVEL. soundings on the seafloor. Sea wall: (1)A structure built along a portion of a coast primarily to prevent EROSION and other The horizontal positions of navigation aids and other important features should be determined to the accuracy stated in Table 2, at 95% confidence level. The summary of minimum standards for 35 8 positioning of navigational aids and important features is given in table 2 below. including data acquisition, processing, storage, distribution, ease of use, and inclusion in the decision making process. Table 2: Summary of Minimum Standards for Positioning of Navigational Aids and Important Features Neap tidal current: Tidal current of decreased velocity occurring semimonthly as the result of the moon being in quadrature. Special Order Order 1 Order 2 and 3 Neap tide: Tide of decreased range occurring semimonthly as the result of the moon being in quadrature. The NEAP RANGE of the tide is the average semidiurnal range occurring at the Surveys Surveys Surveys time of neap tides and is most conveniently computed from the harmonic constants. The NEAP RANGE is typically 10 to 30 percent smaller than the mean range where the type of tide is either Fixed aids to 2m 2m Sm semidiurnal or mixed and is of no practical significance where the type of tide is DIURNAL. navigation and The average height of the high waters of the neap tide is called NEAP HIGH WATER or HIGH features significant WATER NEAPS (MHWN), and the average height of the corresponding LOW WATER is to navigation called NEAP LOW WATER or LOW WATER NEAPS (ML WN). "Natural Coastline 10m 20m 20m Nearshore: (1) In beach terminology an indefinite zone extending seaward from the Mean position of 10m 10m 20m SHORELINE well beyond the BREAKER ZONE. (2) The zone which extends from the swash zone to the position marking the start of the offshore zone, typically at water DEPTHS of the floating aids to order of20 m. Nearshore current: The current system caused by wave action in and near the BREAKER navigation ZONE, and which consists of four parts: the shoreward mass transport of water; longshore Topographical 10m 20m 20m currents; rip currents; and the LONGSHORE movement ofthe expanding heads of rip currents. features Oceanography: That science treating of the oceans, their forms, physical features and phenomena. 2.5 Vertical Controls Offshore: (I) In beach terminology, the comparatively flat zone of variable width, extending 2.5.1 Sounding Datum and Chart Datum from the SHOREFACE to the edge of the CONTINENTAL SHELF. It is continually Sounding datum must always be connected to at least two fixed monuments. Where there is a submerged. (2) The direction seaward from the shore. (3) The zone beyond the nearshore zone land leveling system available, connection to this must be made on shore and offshore platform. where sediment motion induced by waves alone effectively ceases and where the influence of the sea bed on wave action is small in comparison with the effect of wind. (4) The BREAKER Sounding Datum is usually the lowest Astronomical Tide or Low Low Water Spring (LL WS), ZONE directly seaward of the LOW TIDE line. etc. available in the survey area. Ordinary high water mark (OHWM): (SMP) That mark that will be found by examining the bed 2.5.2 Tidal Observation and banks and ascertaining where the presence and action of waters are so common and usual, and so long continued in all ordinary years, as to mark upon the soil a character distinct from that Tidal observations are required: of the abutting upland, in respect to vegitation as that condition exists on June I, 1971, as it may a) for the reduction of soundings; naturally change thereafter, or as it may change thereafter in accordance with permits issued by b) to provide the data for predicting the tide. a local government or the department. Perigean range: The average semidiurnal range occurring at the time of the PERIGEAN In the execution of Hydrographic Survey project, permanent tide gauges should be used for tidal TIDES and most conveniently computed from the harmonic constants. It is larger than the mean observations, where they exist. In this case, it is advisable to confirm that the zero setting of such range where the type of tide is either semidiurnal or mixed and is of no practical significance tide gauges were correctly done. Sounding should be related to local gauges but if this is not where the type oftide is DIURNAL. possible at remote sites a standard datum transfer procedure should be adopted. Locally installed gauges are to be related to sounding datum by means of the same procedure specified above. Perigean tidal currents: Tidal currents of increased velocity occurring monthly as the result of the moon being in perigree (i.e., at the point in its orbit nearest the Earth). The intervals between observation of the height tide is governed by the range of the tide scale. The normal interval is 30 minutes, but if the range is small, hourly readings will suffice. Perigean tides: tides of increased range occurring monthly as the result of the moon being in However, in places where the range is great, intervals of 15 or 20 minutes must be maintained in pengree. order that the tidal curve may accurately reproduce the tidal movement and that errors in the Pier: A structure, usually of open construction, extending out into the water from the shore, to reduction of soundings are minimal. In all cases observations must be taken, at 10-minute serve as a landing place, recreational facility, etc., rather than to afford coastal protection. interval near High or Low waters so as to obtain the times of these occurrences with the greatest possible precision. 9 34 equivalent of a mean 19-year value. All high water heights are included in the average where Tidal height observation for Tidal analysis should extend over the longest possible period and the type of tide is either semi diurnal or mixed. Only the higher high water heights are included not less than 29 days at a few points in the area. Tidal heights should be observed with an in the average where the type of tide is DIURNAL. So determined, MEAN HIGH WATER in accuracy of at least 0.1 metre in height and 3 minutes in time. Care must be taken that tidal the latter case is the same as MEAN HIGHER HIGH WATER. observations are obtained for each of the tidal regimes which may occur within the area being Mean high water springs (MHWS): The average height of the high water occurring at the sounded. time of spring tides. Tidal heights should be observed so that (he total measurement error at the tide gauge, Mean lower low water (MLLW): The average height of the lower low waters over a 19-year including timing error, does not exceed +/- 5 em at 95% for Special Order surveys. For other period. For shorter periods of observation, corrections are applied to eliminate known surveys +/- 10 em should not be exceeded. variations and reduce the result to the equivalent of a mean 19-year value. Mean low water (MLW): The average height of the low waters over a 19-year period. F or 2.6 DATA ACOUISITION shorter periods of observation, corrections are applied to eliminate known variations and 2.6.1 Sounding reduce the result to the equivalent of a mean 19-year value. Mean low water springs (MLWS): The average height ofthe low waters occurring at the time 2.6.1.1 Requirement of the spring tides. The purpose of sounding must always be stated, whether for charting, laying of pipeline, etc. Mean range of tide: The difference in height between MEAN HIGH WATER and MEAN LOW WATER. 2.6.1.2 Characteristics Mean rise ofthe tide: The height of MEAN HIGH WATER above the plane of reference or The Characteristics of the equipment must always be stated i.e. frequency, beam width etc. DATUM of chart. 2.6.1.3 Calibrations Mean sea level: The average height of the surface of the sea for all stages of the tide over a The echo-sounder must be calibrated before and after the day's work. Method of calibration 19year period, usually determined from hourly height readings (see sea level datums). must be stated. Use of temperature/salinity depth probes to determine theoretical values of Mean tide level: Same as HALF -TIDE LEVEL. speed of propagation, velocity meters or bar checks. Mean water level: The mean surface level as determined by averaging the heights of the water at equal intervals of time, usually at hourly intervals. 2.6.1.4 Depth Measurement Mean wave period: The mean of all individual waves in an observation interval of Determination of the general seabed topography, tidal reduction, and detection, classification approximately half an hour. and measurement of seabed hazards are fundamental hydrographic surveying task-. Depths Metadata: Information describing characteristics of data, e.g. the accuracy of survey data. above hazards need to be determined with, at least, a depth accuracy as specified for Order 1 in ISO definition: Data (describing) about a data set and usage aspect of it. Metadata is data Table 1. For wrecks and obstructions which may have less than 40 m clearance above them and may be dangerous to normal surface navigation, the least depth over them should be implicitly attached to a collection of data. Examples of metadata include overall quality, data determined either by high definition sonar examination or physical examination. (diving). set title, source, positional accuracy and copyright. Mechanical sweeping may be used when guaranteeing a minimum safe clearance depth. Mixed current: Type of tidal current characterized by a conspicuous velocity difference All anomalous features previously reported in the survey area and those detected during the between the two floods or two ebbs usually occurring each tidal day. See also MIXED TIDE. survey should be examined in greater detail and, if confirmed, their least depth be determined. Mixed tide: Type of tide which the presence ofa DIURNAL wave is conspicuous by a large The agency responsible for survey quality may define a depth limit beyond which a detailed inequality in either the high or LOW WATER heights with two high waters and two low waters seafloor investigation, and thus an examination of anomalous features, is not required. usually occurring each tidal day. In strictness, all tides are mixed, but the name is usually Measured depths should be reduced to chart or survey datum, by the application of tidal or applied without definite limits to the tide intermediate to those predominantly semi diurnal water level height. Tidal reductions should not be applied to depths greater than 200 m, except and those predominantly DIURNAL. when tides contribute significantly to the Total Propagation Error (TPE). Mole: In coastal terminology, a massive solid-filled structure (generally revetted) of earth, 2.6.1.5 Accuracy masonry or large stone. Depth accuracy is to be understood as the accuracy of the reduced depths. In determining the National Spatial Data Infrastructure (NSDI): This is a nationwide effort to improve the depth accuracy, the sources of individual errors need to be quantified. All error sources should: utilization of geospatial data. The effort is for the benefit of local and state coastal resource be combined to obtain a Total Propagated Error (TPE). TPE results from the combination of all 33 10 contributing errors which include among other things: navigation. (2) (SMP) A structure usually projecting out into the SEA at the mouth of a river for the a) measurement system and sound velocity errors purpose of protecting a navigational channel, a harbor or to influence water currents. b) tidal measurement and modelling errors, and Knot: The unit of speed used in navigation. It is equal to onenautical mile (6076.115 feet or 1852 c)data processing errors. meters) per hour. A statistical method for determining depth accuracy by combining all known errors should be adopted and checked Lagging of tide: The periodic retardation in the time of occurrence of high and LOW WATER due The TPE, determined statistically at the 95% confidence level, is the value used to describe the to changes in the relative positions of the moon and sun. See DAILY RETARDATION OF TIDES. depth accuracy achieved. The TPE should be recorded together with the sounding value. Lagoon: A shallow body of water, as a pond or lake, which usually has a shallow restricted INLET Recognizing that there are both constant and depth dependent errors that affect the accuracyof from the sea. See Figure 5. depths, the formula under Table 1 in Chapter 1 is to be used to compute, at 95% confidence level, Line of position (LOP). A line indicating a series of possible positions of a craft, determined by the allowable depth errors by using for a and b the values from row 3 of Table 1. observation or measurement. Also called position line. (IHO S32 ed.1994, #2848) Specific allowable errors in accuracy of depth measurement. Total error in measuring depths shall Littoral: (1) Of, or pertaining to, a shore, especially a seashore. (2) (SMP) Living on, or occurring with a probability of at least 50%, not exceed: on, the SHORE. a) 0.3metresfrom 0 to 30metres Littoral currents: A current running parallel to the BEACH and generally caused by waves b) 1 % of depths greater than 30 metres. striking the shore at an angle. Measured depths should be reduced to the sounding datum by the application of tidal height. The Littoral zone: An indefinite zone extending seaward from the shoreline to just beyond the error of such reductions should not exceed the errors acceptable for depth measurement specified BREAKER ZONE. above. Ocean depths greater than 200 normally need not to be reduced for tidal height. Over a flat Longshore: Parallel and close to the COASTLINE. or gently sloping bottom a difference in depth at the intervention of two crossing lines of soundings Longshore current: A current located in the surf zone, moving generally parallel to the shoreline, which exceed two times the relevant value given above should be investigated. Such discrepancy generated by waves breaking at an angle with the shoreline, also called the ALONGSHORE may be due to a correctable error in position, sounding, or tidal reduction. In rugged areas it will be CURRENT. See also NEARSHORE CURRENT SYSTEM). See Figure 6. necessary to look for systematic differences persisting over several intersections. For the determination ofleast depth over wrecks and obstruction. For marks and obstruction which may Longshore drift: Movement of sediments approximately parallel to the COASTLINE. have depths less than 40 metres and may be dangerous to navigation, the least depth over them Lower High Water (LHW): The lower of the two high waters of any tidal day. See Figure II. should, whenever possible be determined by either physical examination by diving, by wire Lower low water (LLW): The lower of the two low waters of any tidal day. The single LOW sweeping or by high definition sonar. The standards of accuracy as specified above should be WATER occurring daily-during periods when the tide is DIURNAL is considered to be LLW. See achieved where equipment permits. Figure II. 2.6.1.6 Interval Between Sounding Lines Lower low water datum: An approximation to the plane of MEAN LOWER LOW WATER that An appropriate line spacing for the various orders of survey is proposed in Table 1. The interval has been adopted as a standard reference plane for a limited area and is retained for an indefinite period regardless of the fact that it may differ slightly from a better determination of MEAN between sounding lines should be determined having regard to the significance of the area and to LOWER LOW WATER from a subsequent series of observations. Low tide: See LOW WATER the topographical character of the sea floor, water depth, the coverage provided by the sounder and the means available for searching between lines. In principle, the interval between principal' Low water (LW): The minimum height reached by each falling tide. Nontechnically, also called sounding lines should not be more than lflmrh at the scale of the survey. This recommend interval LOW TIDE. must be reduced where the sea floor is abnormally irregular and may be increased when multibeam Low water line: The line where the established LOW WATER DATUM intersects the shore. The echo-sounder or means of searching for anomalies between lines are in use. All anomalous depths plane of reference that constitutes the LO W WATER DATUM differs in different-regions. previously reported in the survey area and those detected during the survey should be examined in Mean higher high water (MHHW): The arithmetic average of the elevations of the higher high greater detail and if confirmed, the least depths over them determined. Whenever practicable waters of a mixed tide over a specific 19year period. For shorter periods of observation, anomalous depths reported near the survey area should also be examined. corrections are applied to eliminate known variations and reduce the result to the equivalent of a Cross check lines normal to the principal sounding lines should always be run whenever sea bed mean 19-year interval. conditions make it possible to confirm, by this method, the accuracy of positioning, sounding and Mean high water (MHW): The average ELEVATION of all high waters recorded at a particular tidal reductions. The interval between cross-check lines should normally be no more than 1.5 point or station over a considerable period of time, usually 19 years. For shorter periods of observation, corrections are applied to eliminate known variations and reduce the result to the 11 32 Great diurnal range: The difference in height between MEAN HIGHER HIGH WATER times t at 0 t e pnncipa soun mg meso and MEAN LOWER LOW WATER. The expression may also be used in the contracted 2.6.1. 7 Interval Between Plotted Soundings form diurnal range. Sounding plotted, along principal sounding lines should be selected giving priority to peaks, deeps Groin: (I) A shore-protection structure(built usually to trap LITTORAL DRIFT or retard and points of change in slope. Intermediary soundings should then be selected at intervals not EROSION of the shore). It is narrow in width (measured parallel to the shore)and its length exceeding 5mm at the scale of surveys, except where the sea-bed is even, when the interval may be may vary from tens to hundreds of meters (extending from a point landward of the increased to IOmm. shoreline out into the water).GROINS may be classified as permeable(with openings through them)or impermeable (a solid or nearly solid structure). (2( (SMP) )A barrier-type 2.6.1.8 Spacing of Position Fixes structure extending from the BACKSHORE or stream bank into a water body for the The interval between position fixed on the survey sheet shall in principal be no greater than purpose of the protection of a SHORELINE and adjacent upland by influencing the 40mm. If the vessel is steered on an arc, the interval should be reduced to permit accuracy in movement of water and/or deposition of materials. plotting intermediary soundings. Half-tide level: A plane midway between MEAN HIGH WATER and MEAN LOW WATER, 2.6.1.9 Recommended Tracks also called MEAN TIDE LEVEL. Every track recommended for navigation should be sounded along and preferably sonar swept by Harbor: A water area nearly surrounded by land, sea walls, BREAKWATERS or artificial dikes, either a multi-beam or side scan or a high definition sector-scanning sonar to ensure complete forming a safe anchorage for ships. coverage of the track and its adjacent area. In areas not covered by a standard large scale Heave: (1) The vertical rise or fall of the waves or the sea. (2) The translational movement of a craft hydrographic survey, at least three combined sounding and sonar lines should be run; one along parallel to its vertical axis. (3) The net transport of a floating body resulting from wave action. the center line and one on each side or the track. Higher high water (HHW): The higher of the two high waters of any tidal day. The single high water 2.7 Sonar Sweeping occurring daily during periods when the tide is DIURNAL is considered to be HIGHER HIGH Sonar sweeping is required to detect all protuberances from the sea -bed which could be a danger to WATER. See Figure 11. shipping, or to engineering works by means of reflection of sonar beams transmitted sideways Higher low water (HLW): The higher of the two low waters of any tidal day. See Figure 11. from a vessel. Delineation of areas of sand waves, surface rock outcropping pockmarks or other features. Scouring or cover of pipelines, foundations, etc. High water (HW): Maximum height reached by a rising tide. The height may be solely due to the periodic tidal forces or it may have superimposed upon it the effects of prevailing meteorological In Sonar Surveys a plot of the fixes used to control the work is required together with an overlay conditions. Nontechnically, also called the HIGH TIDE. indicating the areas covered by the search and the results thereof in plan. Scale of th~ plot is the High water line: The intersection of MEAN HIGH WATER with the shore. The shoreline delineated same as. the one for sounding. on the nautical charts ofthe U.S. Coast and Geodetic Survey is an approximation of the mean high waterline. 2.7.1 Sonar Records All records from Sonar Surveys shall be deposited in the Data Centre of the Hydrographer of High water mark: A reference mark on a structure or natural object, indicating the maximum stage the Navy. They are to include a full interpretation report covering all aspects of the survey of tide or flood. including: Hydrography: (1) The description and study of seas, lakes, rivers and other waters. (2) The science of locating aids and dangers to navigation. (3) The description of physical properties of the Horizons encountered waters of a region. Times Scales Correlation with bore-hole results Inshore: (1) The region where waves are transformed by interaction with the sea bed. (2) In beach Technical operation report terminology, the zone of variable width extending from the LOW WATER LINE through the Sonar contact interpretation BREAKER ZONE. Horizon plot Inshore current: Any current inside the SURF ZONE. Profile, and Intertidal: The zone between the high and LOW WATER marks. With firm conclusions. Isobath: Line connecting points of equal water DEPTH on a chart; a seabed contour. 2.8 Bottom/Water Sampling Jetty: (1) On open seacoasts, a structure extending into a body of water to direct and confine the The nature of the seabed should be determined by sampling or may be inferred from other sensors stream or tidal flow to a selected CHANNEL, or to prevent shoaling. Jetties are built at the mouth of (e.g. single beam echo sounders, side scan sonar, sub-bottom profiler, video, etc.) up to the depth required by local anchoring or trawling conditions; under normal circumstances sampling is not a river or ENTRANCE to a BAY to help deepen and stabilize a CHANNEL and facilitate 31 12 required in depths greater than 200m. Samples have to be spaced according to the seabed a continuously running ebb current, the velocity alternately increasing and decreasing without geology. Spacing of samples should normally be 10 times that of the selected line spacing. In coming to a slack or reversing. The expression MAXIMUM EBB is also applicable to any EBB areas intended for anchorages, density of sampling should be increased. Any inference technique CURRENT at the time of greatest velocity. should be ground-truthed by physical sampling. Echo sounder: An instrument for determining the DEPTH of water by measuring the time of The sample is bagged in an air-tight container and marked with sufficient data to identify it by travel ofa sound-pulse from the surface ofa body of water to the bottom and back. date, survey position and water depth. The bagged samples are forwarded to the laboratory for EEZ: This means Exclusive Economic Zone and is related to 200km of the continental shelf. grain-size analysis. Gravity cores are taken at specific intervals by any proven method. Error: The difference between an observed or computed value of a quantity and the ideal or true value ofthat quantity. (IHO S32 ed.1994, # 1671) 2.8.1 Magnetometer Search Escarpment: A more or less continuous line of CLIFFS or steep slopes facing in one general Magnetometer search for underground cables, pipes and other materials beneath the sea bed direction which are caused by EROSION or faulting, also called SCARP. should be made during the course of hydrographic surveys. Fairway: The parts of a waterway kept open, and unobstructed, for navigation. 2.9 Rendering of Data Fathom: A measure of water DEPTH equal to 1.83 m(6 feet). The format for presenting the report of a survey shall be as follows: Feeder beach: An artificially widened beach serving to nourish DOWNDRlFT beaches by a) Title Page natural LITTORAL CURRENTS or other forces. b) Location MaplDiagram (Digi tal format) Fetch: The length of unobstructed open sea surface across which the wind can generate waves c) Table of Contents (GENERATING AREA). d) Sections i) Summary of Results Floodplain: (1) A flat tract ofland bordering a river, mainly in its lower reaches, and consisting of ii) Introduction ALLUVIUM deposited by the river. It is formed by the sweeping of the meander belts iii) Description of the delibration, operation and events downstream, thus widening the valley, the sides of which may become some kilometers apart. In iv) Survey equipment, methods and procedure time of flood, when the river overflows its banks, sediment is deposited along the valley banks v) Data reduction and results and plains. (2) (SMP) Synonymous with 100-year floodplain. The land area susceptible to being vi) Appendices/Enclosures inundated by stream derived waters with a 1 percent chance of being equaled or exceeded in any i) Tidal information/Reduction given year. The limits of this area are based on flood regulation ordinance maps or reasonable ii) Calibration Reports method that meets the objectives of the SMP (WAC 173-22-030(2). iii) Charting Fully-developed sea: The waves that form when wind blows for a sufficient period of time across iv) Survey log sheets the open ocean. The waves of afully developed sea have the maximum height possible for a given v) Enclosures wind speed, FETCH and DURATION of wind. vii) List of Personnel Gabion: (1) Steel wire-mesh basket to hold stones or crushed rock to protect a BANK or bottom from EROSION. (2) (SMP) Structures composed of masses of ROCKS, rubble or masonry held 2.10 Draughting tightly together usually by wire mesh so as to form blocks or walls. Sometimes used on heavy All draughting shall be undertaken on "A" series sizes sheets. All originals shall be prepared on erosion areas to retard wave action or as a foundation for BREAKWATERS or JETTIES. stable transparent film materials. The title block shall be on the right hand side of the chart. A one Geographic Information System (GIS): A system of spatially referenced information, including millimeter wide line shall surround both the title block and-the main. drawing which are separated computer programs that acquire, store, manipulate, analyze, and display spatial data. by an 8mm space. Minimum interval with of the title block shall be 150mm. ' Geoid: The equipotential surface of the Earth's gravity field which best fits, in the least squares The title block shall be further divided into boxes separated by a I mm thick level and shall contain sense, MEAN SEA LEVEL. the following: Geostatistics. The field of statistics which deals with estimating the confidence of interpolated a) Title e.g. Nigerian Navy (with Logo)' values derived from measurements of geo-referenced data. b) Location Chart (Location chart in relationto other union features) c) Geodetic information (Spheroidand'ellipsoid datum, e.g. TN Mangnetic variation of Global Positioning System (GPS): A navigational and positioning system developed by the U.S. area).. Department of Defense, by which the location of a position on or above the Earth can be d) Legend - Systems used e.g. Transponder, Syledis GPS etc. (Symbols used on the determined by a special receiver at that point interpreting signals received simultaneously from chart). several of a constellation of special satellites. e) Scale (Scale of the chart). 13 30 descent toward the great depths. (2) The area under active LITTORAL processes during the (t) Notes (Notes on remarks concerning the chart) Holocene period. (3) The region of the oceanic bottom that extends outward from the shorel ine with (g) Interpretation and Draughting (Name, Signature and Date of Surveyor, Officer i/c for an average slope of less than 1:100, to a line where the GRADIENT begins to exceed I :40 (the final approval of the job). Chart number, Date of Survey vessels used, Ref. No. CONTINENTAL SLOPE). (h) Name ofContractor(Contractor's logo-Name and Address) Continental slope: The declivity from the offshore border of the CONTINENTAL SHELF to 2.11 Chart drawing oceanic depths. It characterized by a marked increase in slope. (a) Easting, Northing, Latitude and longitude values shall be marked on the grid of Controlling depth: The least DEPTH in the navigable parts of a water way, governing the graticules at the extreme limits on the chart. maximum draft of vessels that can enter. (b) Bathymetric charts shall contain, where possible all corrected sounding taken. Control network: Geodetic control together with the measure or adjusted of the distance, angles, The soundings shall be rounded to the nearest 0.1 metres with the decimal point directions, or heights used in determining the coordinates of the control. being the actual plotted position. Correction: A quality which applied to an observation or function thereof will diminish or (c) All survey data shown on the charts shall be drawn in such a way as to ensure that eliminate the effects of errors and give an improved value of the observation or function. The no other data is obscured by it. correction corresponding to a given error of the same magnitude but of opposite sign (lHO S32 ed. (d) As appropriate charts shall be clearly marked. Preliminary, Provisional or 1994, # 1 079) Fair, e.t.c Countercurrent: A secondary current usually setting in a direction opposite to that of a Marin current. (e) All other drawing shall be an acceptable tracing film. Current: (1) The flowing of water or other liquid or gas. (2) That portion of a stream of water which 3.0 DREDGING is moving with a velocity much greater than the average or in which the process of the water is 3.1 Hydrographic Survey Specification for Dredging principally concentrated. Three aspects are considered for dredging viz: Datum: Any position or element in relation to which others are determined, as datum point datum (I) Pre- Dredging Surveyor Dredging feasibility study; line, DATUM PLANE. See Figure 11.